fix: [Python] consistent mangled names for generic abstract class members

[dbrattli]

Summary

Derived classes of generic abstract classes could not be instantiated in compiled Python due to mismatched mangled method names between the abstract stub and the override.

Root cause

Two different name-mangling paths:

• Abstract stubs (Fable2Python.Bases.fs, InterfaceNaming.getMangledMemberName) derived the entity prefix from memb.FullName, which from the F# compiler does not include the backtick generic arity. The code also replaced . with _ but left backticks untouched.
• Overrides in derived classes (FSharp2Fable.Util.fs, getMangledAbstractMemberName) built the prefix from the entity's own TryFullName, which does include the arity (e.g. `2), and downstream sanitization in Python.Prelude.sanitizeIdentForbiddenChars replaced both . and ` with _.

Result: for TestRunner<'A, 'B> the abstract stub was named TestRunner_get_Encode while the override was named TestRunner_2_get_Encode. Python's ABC then refused to instantiate any derived class:

TypeError: Can't instantiate abstract class PythonTestRunner without an implementation for abstract methods
  'Thoth_Json_Tests_Testing_TestRunner_get_Decode',
  'Thoth_Json_Tests_Testing_TestRunner_get_Encode',
  ...

Fix

getMangledMemberName now uses ent.FullName (entity full name with arity) and Fable.Py.Naming.cleanNameAsPyIdentifier, which replaces both . and ` with _. This matches the mangled name that derived-class overrides produce.

For non-generic abstract classes the output is unchanged.

Test plan

• New regression test Generic abstract class can be instantiated through derived class in tests/Python/TestType.fs covering an abstract method, an abstract property, and an abstract generic mapper.
• Full tests/Python suite passes locally (2331 tests).
• Verified externally against Thoth.Json's Python tests (239 tests, previously broken on this branch, now green).

🤖 Generated with Claude Code

[github-actions]

Python Type Checking Results (Pyright)

Metric	Value
Total errors	33
Files with errors	4
Excluded files	4
New errors	✅ No

Excluded files with errors (4 files)

These files have known type errors and are excluded from CI. Remove from pyrightconfig.ci.json as errors are fixed.

File	Errors	Status
temp/tests/Python/test_hash_set.py	18	Excluded
temp/tests/Python/test_applicative.py	12	Excluded
temp/tests/Python/test_nested_and_recursive_pattern.py	2	Excluded
temp/tests/Python/fable_modules/thoth_json_python/encode.py	1	Excluded